Various electrical apparatus such as circuit breakers, motor controllers, monitors, motor starters have many common features. Microprocessor-based overcurrent protection devices such as circuit breakers and motor overload relays must measure electrical current and then produce a time-to-trip versus current characteristic curve should the current be above a preset value. Basically, these apparatus rely upon sensing electrical variables such as line to neutral voltage, line-to-line voltage, phase current, frequency and the like as inputs, either alone or in combination, for ultimately causing a desirable electrical function to occur. For instance, circuit breakers sense electrical current and open upon the occurrence of a predetermined amount. Motor controllers are utilized to start motors to relay information about the status of the motor from one station to another, etc. Monitors provide readout information about the status of line currents, voltages, power, frequency, etc. The state of the art has advanced to a position where the aforementioned may provide current, voltage, power, and frequency information, etc. to a printed circuit card which may have surface mounted components, discrete components, transformers, inductors, capacitors, operational comparators and all of the appropriate interconnecting paths as well as personal computer printed circuit (PC) devices such as communication networks, analog-to-digital (A-to-D) converters, counters, etc. disposed thereupon. In many applications, microprocessors having random access memories (RAM) and electrically erasable programmable read only memories (EEPROM) are also included. Part or all of these may be contained in large scale integrated circuits or combinations of large scale integrated circuits and discrete components. Many of the aforementioned contained potentiometer devices such as three-point or two-point variable resistors, variable capacitors, adjustable operational comparators, etc.
It has long been recognized that in order for any of the devices or apparatus described previously to operate reliably and accurately, it is necessary for the sensing transformers, internal circuit board components, large scale integrated circuits, etc. to accurately depict and react to the basic circuitry and at output devices such as readout devices, circuit breaker tripping devices, relaying devices, those variables which are monitored or read. For example, the measurement of current usually requires a calibration procedure as the "as built" accuracy of circuitry used to convert the analog current values to digital values usually is not precise enough. For instance, if a circuit breaker is programmed to trip at a specific period of time at 10 amperes, for example, and if in fact 10 amperes of current are flowing in the line to be protected but gain adjustment and offset factors within the circuitry make it appear that only 9.9 amperes of current are flowing, then an error exists which could lead to catastrophic consequences. In the past, provision has been made for eliminating the error by placing the device in question in a calibration mode whereby a precision value of a desired variable is provided as a sensed input to the system and various potentiometers, offset adjustment devices and the like are manipulated or "tweaked" at the end of the production process so that desirable occurrences happen at the exact value of input variable at which they are supposed to happen. Generally, after this has been completed, a protective coating of material, a "conformal coating", is placed over the adjustment devices so that they may not be tampered with by subsequent purchasers, users, etc. This leads to a number of disadvantages or problems. One disadvantage lies in the fact that the process is highly labor intensive and furthermore requires a great deal of judgement, experience and perhaps even dexterity on the part of the calibrator or adjustor. Furthermore, in some instances where a microprocessor is employed, a resistive-capacitive network rather than a crystal oscillator may be utilized for determining the microprocessor's time base. In sensitive applications, this time base accuracy may not be sufficient. In addition, problems can arise if the board to be calibrated is small.
Two areas in which calibration problems are likely to arise are associated with analog sampling systems that require "gain" and exhibit "offset". Conventional calibration procedures with the use of trim pots and component selection are not only expensive but can be difficult to implement, especially if the board is very small and/or has been conformal coated. Ideally the trip unit should be calibrated without physically modifying the printed circuit board. Gain adjustments are required for precision circuits requiring operational comparators to magnify or attenuate a signal before being processed by an analog-to-digital converter. Offset adjustments are made to remove errors caused by DC biasing currents existing in a circuit. Those offsets could be present on the input signals or generated by operational comparators in the circuit. Gain adjustment is usually made by imposing a multiplying factor on a signal whereas offset adjustment is made by adding or subtracting a constant to or from the signal being processed. U.S. Pat. No. 4,550,360 issued on Oct. 29, 1985 to J. J. Dougherty, entitled "Circuit Breaker Static Trip Unit Having Automatic Circuit Trimming", teaches one way to overcome problems associated with the prior art. The Dougherty patent describes the use of a microprocessor and selected input values to correct for gain and offset. However, emphasis is directed to correction for individual components or a class thereof within the system. It requires testing individual components or classes of individual components using different inputs such as, for example, full scale current for current transformers and five milliamperes for a diode and then deriving a microprocessor memory correction value related thereto. However, it would be advantageous to be able to calibrate the entire system using only a single input where the calibration is related to the ultimate output value rather than to the individual values of components. It would be further advantageous to be able to achieve the foregoing without adding extra component elements to the system being calibrated and to do the calibration in a non-obtrusive manner. It can be seen, therefore, that the prior art mode for calibration has many disadvantages as described previously. It would be desirous therefore if an advantageous calibration procedure for apparatus could be found which was relatively inexpensive, required a minimum of human intervention, was highly accurate, highly reliable and as close to being automated as possible.